When an automobile engine is assembled at the factory, the tolerances for valves, including both the valve guide and valve stem, is very tight. Therefore, the parts utilized in the manufacturing environment can be somewhat standardized with the standards yielding very tight tolerances.
When assembling a valve into the engine, it is important that the valve stem fit within predefined tolerances in the valve guide. In addition, a valve seal is provided on the upper end of the valve stem to minimize the amount of oil that runs down the valve stem into the cylinder bore. This can either be a positive seal or an umbrella valve seal. The positive, this valve seal is mounted over the upper end of the valve guide and secured thereto, and provides a wiping action on the valve stem itself. To insure proper operation, the tolerances of all parts involved in the valve assembly are very precise, which tolerances can easily be accommodated in the manufacturing environment. By comparison, the umbrella seal is mounted on the valve stem and reciprocates with the valve.
When an engine is rebuilt, most of the parts in the valve assembly, including the valve guide, the valve stem, etc., are worn and typically must be replaced. Depending upon the wear, utilization of different size valve stems, valve guides, etc. will be required. If the valve assembly were reassembled in the manner in which it was done at the factory, this would require matching tolerances of all the new parts. For example, if a new valve were utilized having a larger diameter valve stem, this would necessitate matching it with a new valve guide of corresponding dimensions, and also require a valve seal of corresponding dimensions. Depending upon the wear involved in the engine to be rebuilt, any one of a plurality of valve dimensions could be utilized. This presents a problem to a rebuilder in that he would be required to stock a large number of parts to accommodate the varying tolerances.
One technique for minimizing the number of parts stocked by a rebuilder is to utilize "universal" parts that can accommodate varying tolerances. This is especially so with respect to the valve seal. One type of seal that has been utilized in rebuilding is that called the "umbrella seal". In normal manufacturing of a new engine, the valve seal is placed down on the valve guide secured thereto to provide a wiping action. The umbrella seal, in contradistinction, is disposed on the valve stem itself and rides up and down with the reciprocating action of the valve stem. The opening in the umbrella seal is made resilient such that it will expand and accommodate a number of different diameters of valve stems. One problem that has been noted with respect to present umbrella seals is that the resilient materials from which they are made wear out as a function of use and heat, and also due to swelling in the presence of oil. Further, the more resilient the umbrella seal is to accommodate various size valve stems, the poorer the quality is. However, to solve this problem by making the umbrella seal more rigid, additional stock is needed to accommodate the various sizes, as a single size only accommodates a limited range of valve stem diameters.